(Health.com) -- Affluent countries, including the U.S., tend to have higher rates of depression than lower-income nations such as Mexico, a new study from World Health Organization researchers suggests.

In face-to-face interviews, teams of researchers surveyed nationally representative samples of people in 18 countries on five continents -- nearly 90,000 people in all -- and assessed their history of depression using a standardized list of nine criteria.

In addition to looking at personal characteristics such as age and relationship status, the researchers divided the countries into high- and middle-to-low income groups according to average household earnings.

The proportion of people who have ever had an episode of clinical depression in their lifetime is 15% in the high-income nations and 11% in lower-income countries, the study estimates.

France (21%) and the United States (19%) had the highest rates, while China (6.5%) and Mexico (8%) had the lowest.

It's not clear what accounts for this pattern, says Evelyn Bromet, Ph.D., the lead author of the study and a professor of psychiatry and behavioral science at Stony Brook University, in Stony Brook, New York. But she stresses that wealth -- and happiness -- are relative concepts.

"Wherever you are, there's always people doing better than you," Bromet says. "You'd think that countries that are better off should have lower rates [of depression], but just because they have a high income doesn't mean there isn't a lot of stress in the environment."

Moreover, she adds, the richest countries in the world also tend to have the greatest levels of income inequality, which has been linked to higher rates of depression as well as many other chronic diseases.

The income-related trends did not hold for all measures of depression, however. When Bromet and her colleagues looked only at episodes of depression that occurred in the previous year, the rate was nearly identical in higher- and lower-income countries, about 6%. (Here again, though, the U.S. came out close to the top: Its 8% rate was second only to Brazil's 10%.)

This may reflect actual differences in depression rates, but it could also be that people in poorer countries are for some reason less likely to recall or relate episodes of depression from their past, the authors say.

Comparing depression rates across different countries is inherently challenging, because survey participants may be influenced by cultural norms or their interactions with the interviewer, says Timothy Classen, Ph.D., an assistant professor of economics at Loyola University Chicago who has studied the link between economics and suicide.

"There are significant disparities across countries in terms of the availability and social acceptance of mental health care for depression," says Classen, noting that there tends to be more stigma surrounding depression in a country like Japan than in the U.S. (Classen says this may explain why Japan has a higher suicide rate, even though its depression rates in the study were three to four times lower than those in the U.S.)

Different age groups appeared to fare better than others depending on a country's level of affluence. For instance, older adults in high-income countries generally had lower rates of depression than their younger counterparts, while the trend was reversed in several poorer countries.

In a country like the Ukraine, Bromet says, older people "have enormous pressure on them and they don't have enough money to live and take care of grandchildren and health problems. Their lives are extremely difficult relative to older people in this country."

Bromet says the study findings can help countries identify their own high-risk populations, whether it's older adults in Ukraine or young divorced women in Japan.

"I hope people in these countries will start thinking about social and medical support for these groups in particular, and what they can do to prevent depression in the future," she says.

The study, which was published today in the journal BMC Medicine, is part of the WHO's Mental Health Survey Initiative.

Government organizations (including the U.S. National Institute of Mental Health), charitable foundations, and pharmaceutical companies across the world have all helped finance the initiative, but the funders played no role in the data collection, analysis, or publication.